1. Field of the Invention
The invention relates to an improved fuel injection system for an internal combustion engine.
2. Description of the Prior Art
For better understanding of the description and the claims, several terms will first be explained: The fuel injection system of the invention may be either stroke-controlled or pressure-controlled. Within the scope of the invention, a stroke-controlled fuel injection system is understand to mean that the opening and closing of the injection opening is effected with the aid of a displaceable nozzle needle by means of the hydraulic cooperation of the fuel pressures in a nozzle chamber and in a control chamber. A pressure reduction inside the control chamber causes a stroke of the nozzle needle. Alternatively, the deflection of the nozzle needle can be done by means of a final control element (actuator). In a pressure-controlled fuel injection system according to the invention, the nozzle needle is moved by the fuel pressure prevailing in the nozzle chamber of an injector, counter to the action of a closing force (spring), such that the injection opening is uncovered for an injection of the fuel out of the nozzle chamber into the cylinder. The pressure at which fuel emerges from the nozzle chamber into a cylinder is called the injection pressure, while system pressure is understood to mean the pressure at which fuel is available or is stored inside the fuel injection system. Fuel metering means delivering fuel to the nozzle chamber by means of a metering valve. In combined fuel metering, one common valve is used to meter various injection pressures. In the unit fuel injector (PDE), the injection pump and the injector form a unit. One such unit per cylinder is built into the cylinder head and driven by the engine camshaft, either directly via a tappet or indirectly via a tilt lever. The pump-line-nozzle system (PLD) operates by the same method. In this case, a high-pressure line leads to the nozzle chamber or nozzle holder.
For introducing fuel into direct-injection diesel engines, both pressure-controlled and stroke-controlled injection systems are known. To reduce emissions, the highest possible maximum injection pressure and a linear pressure increase are favorable. Combined unit fuel injector and pump-line-nozzle systems PDE/PLD are therefore often used, which make a high injection pressure possible.
It has also proved advantageous if the injection pressure is independent of the engine rpm and load and can be adjusted variably in the performance graph. Multiple injection is also advantageous. Some engine manufacturers therefore employ common rail systems (CRSs).
To improve the function of a PDE/PLD injection system, a stroke-controlled injector may be used. As a result, in the pumping region of the cam, a multiple injection (preinjection, main injection, postinjection) can be realized. For realizing a multiple injection, a lengthened cam stroke and pump stroke are therefore needed. Moreover, upon triggering a postinjection at high pressure, major superelevations of pressure occur, which can destroy the injection system. A postinjection is therefore possible only at low injection pressure. Moreover, no injection outside the cam pumping region is possible, which is important for a widely spaced postinjection for exhaust gas posttreatment systems.